ellsworth



1956 c. E- ELLSWORTH DIELECTRIC HEATING APPLICATQR 4 Sheets-Sheet 1 Filed May 28, 1953 2 I e e h s v e e h s 4 Filed May 28, 1953 Jan. 24, 1956 c, E. ELLSWORTH 2,7

DIELECTRIC HEATING APPLICATOR 4 Sheets-Sheet 3 Filed May 28. 1953 OE .rOZMDOUEm B E M B E H D OPERATING RANGE ELECTRODE HEIGHT-INCHES 0 w 0 O O O O O O O O O O O O W S 7 w 5 4 3 2 l Jan. 24, 1956 c, E. ELLSWORTH 2,732,473

DIELECTRIC HEATING APPLICATOR Filed May 28, 1953 4 Sheets-Sheet 4 United States Patent DIELECTRIC HEATING APPLICATOR Carl E. Ellsworth, Anchorage, Ky., assignor to National Cylinder Gas Company, Chicago, Ill., a corporation of Delaware Application May 28, 1953, Serial No. 358,051

13 Claims. (Cl. 219-1055) This invention relates to high-frequency dielectric heating systems in which an oscillator supplies high-frequency power to dielectric loads disposed between spaced heating electrodes, and has for an important object the provision of improved apparatus for enhancing the operational characteristics of high-frequency applicators, and more particularly apparatus, capable of reducing variations of frequency and electrode voltage with variations in electrode spacing so as to maintain the frequency and elec trode voltage substantially constant throughout a relatively wide operating range.

Heating applicators of the type including variably spaced heating electrodes are subject to changes in electrode voltage arising from variations in electrode height. These heating applicators may be, by way of example, comprised of high-frequency resonant tank circuits in which the heating" electrodes provide substantially all the capacitance. As the spacing between the electrodes is varied to accommodate work loads of varying size, the capacitance and frequency of the resonant tank circuit will change accordingly and ultimately may result in an undesirable change in electrode voltage and the introduction of undesirable harmonics.

This variation in electrode voltage may be minimized as described and claimed in copending Warren application Serial No. 138,628, filed January 14, 1950, now abandoned in favor of Warren's continuation-impart application, Serial No. 419,633, filed March 26, 1954. The present invention provides an improvement thereover in structure which results in smoother operational characteristics while minimizing undesirable current concentrations and the possibilities of introduction of undesirable harmonics and undesirable variations in voltage gradients along the electrode.

In accordance with the present invention there is provided an applicator having a pair of heating electrodes provided with supplementary capacitance plates for maintaining the over-all capacitance of the applicator substantially constant and independent from variations in electrode spacing. More particularly, the applicator to which the present invention has been applied includes an electrically conductive housing having horizontal and vertical side walls and in which at least one vertical wall surface is inclined to define an acute angle with the horizontal wall. A heating electrode is mounted within the housing and is provided with contiguous surfaces that are respectively associated with thehorizontal and inclined walls. The variation in total capacitance between the horizontal housing wall, or another electrode and the heating electrode for different spacings of the heating electrode from said horizontal wall is compensated by a change in opposite sense of the capacitance between the inclined wall surface and the associated electrode. surface. In addition, the total capacitance of the applicator is made very large with respect to changes in heating electrode capacitance to minimize the effects thereof.

For a more detailed description of the invention and for further objects and advantages thereof, reference may be had to the accompanying drawings, in which:

Fig. l discloses in perspective a typical heating applicator of the continuous feed-type embodying the present invention;

Fig. 2 illustrates in perspective another view of the heating applicator shown in Fig. l; v

Fig. 3 diagrammatically illustrates a sectional view of the dielectric heating chamber;

Fig. 4 is an enlarged section of a portion of the heating chamber disclosing the heating electrode in different operational positions;

Fig. 5 is a perspective view of a preferred construction of .the applicator electrode embodying the present invention;

Fig. 6 is a graph illustrating the operational characteristics of the applicator; and,

Fig. 7 is a sectional view through the longitudinal portion of the heating applicator, disclosing a damper con trol for maintaining the heating chamber free of vapor.

Referring to the drawings in which like reference char acters designate the same parts throughout the several views and more particularly to Figs. 1 and 2, there is disclosed a high-frequency dielectric heating applicator 10, embodying the present invention. The applicator 10 is shown to be of the continuous feed-type in which work is passed into a heating chamber, located within the center portion of the applicator, by means of a conveyor 11 and by way of an entrance vestibule 12. The work, for example foundry sand cores, continues through the heating chamber and upon being dried is emitted from the applicator through the exit vestibule 13. The applicator is provided with a high voltage D. C. power supply which may be positioned exterior thereto and adjacent the side thereof within a shielded cabinet 14, and an oscillator for providing high-frequency power and which is located in cabinet 15. in the interest of saving space and providing a heating applicator which in appearance is more streamlined, it may be desirable to mount the oscillator and the D. C. power supply cabinets respectively atop the entrance and exit vestibules 12 and 13. This alternate construction is illustrated by dotted lines in Fig. 2.

The heating chamber of applicator 10 is schematically illustrated in Fig. 3 as including an electrically conductive enclosure or housing 20 comprised of horizontal walls 21, 22 and inclined vertical walls 23 and 24. The inclined vertical or side walls 23 and 24 converge in direction away from the horizontal wall 22. An extension of the plane of each inclined wall forms an acute angle with the contiguous horizontal wall 22. The housing 20 may be completely open ended but is preferably a substantially complete enclosure for the purpose of providing among other things a reduction in radiation of the high-frequency fields within its confines.

The upper horizontal wall 21 is electrically connected by a box-fin inductance 27, 27a to the heating electrode structure 25. As thus far described, the housing 20, inductor 27, 27a and the electrode 25 define a reentrant cavity applicator generically similar to that of the aforesaid Warren applications. it is characteristic of the resonator thus formed that its resonant wavelength may be long as compared to its dimensions.

The upper electrode 25 is spaced from the lower electrode, or lower horizontal wall 22. The heating of a load, such as foundry sand cores and the like, takes place within an electric field produced between the two electrodes 22 and 25. Although not specifically disclosed in Fig. 3, a metallic conveyor may serve as the lower heating electrode so as to provide for continuous processing of work. Alternatively, a non-metallic conveyor may be used to pass work through the interelectrode space.

in order to accommodate runs of different sized work and to control oscillator loading, the upper electrode 25 is made movable so as to provide a variation in spacing between it and the lower electrode 22. As the upper electrode 25 is-raised and lowered there is effected a change in interelectrode capacitance which in absence of the invention would be ultimately reflected as a change in frequency and in the voltage between the electrodes. In order to compensate for this change and to thereby minimize changes in frequency and electrode voltage, there is provided a supplementary capacitance which varies in a manner compensatory to changes in capacitance between electrodes 22 and 25 to maintain the ap plicator capacitance substantially constant. This supplementary capacitance is provided by a capacitive surface or skirt 26, mounted on and extending upwardly from the electrode 25, and the inclinedwalls 23 and 24 of the housing. In applicators of the enclosed type, additional capacitance may be derived from the slanting end walls 23a, 24a (Fig. 7) and skirt 26 which would then be comprised of four distinct surfaces each associated with a slanting wall of the housing 20. Although the inclined walls of the housing are shown to comprise surfaces associated with the skirt 26 it will be understood that Within the scope of the present invention, the housing may be comprised of vertical walls and have mounted therein, supplementary inclined surfaces to be associated with the skirt. The walls and skirt may be planar as illustrated, or may be of curved or other configuration, but in any event the skirt 26 is disposed parallel or substantially parallel with and in spaced relation to the inclined walls. This parallel relationship is maintained at all heights assumed by the electrode structure 25 but the spacing varies as now described.

The manner in which the variations in capacitance between the electrodes 25 and 22 is compensated for by the supplementary capacitance of the skirt 26 and the inclined walls is illustrated in Fig. 4. As shown, when the main electrode 25 is in a lower position for which the capacitance between it and electrode 22 is at a maximum, the spacing between skirt 26 and vertical wall 23 is a maximum and the capacitance therebetween is therefore at a minimum. As the electrode 25 is raised and the interelectrode capacitance begins to decrease, the capacitive element or skirt 26 is brought closer to the wall 23 thereby increasing the supplemental capacitance resulting in the over-all capacitance of the applicator being maintained substantially constant. As set forth above, the skirt 26 may define a continuous surface about the entire periphery of the electrode 25 or may if desired be comprised of a single sheet of conductive material mounted on but one side of the electrode. An example of this latter embodiment is illustrated in Fig. 4.

In addition to the effect of the compensatory action of the skirt 26, the frequency and electrode voltage may be stabilized by providing the applicator with total capacitance that is large with respect to anticipated changes in interelectrode capacitance.

The total applicator capacitance is CT=CT1+CTz where Cm is the total capacitance between the heating electrode 25 and its associated electrode, and Crz is the total supplementary capacitance between the skirt 26 and the associated inclined wall or walls.

The total interelectrode or heating electrode capacitance may be expressed as where CM1 is the minimum interelectrode capacitance and AC1 is the change in interelectrode capacitance. The total skirt capacitance may be expressed as At all times CM1+CM2=K where K is a constant.

This sum is preferably made very large with respect to change in interelectrode capacitance, as indicated in the following equation:

With the existing relationship as expressed above in Equation 5 the change in interelectrode capacitance AC1 will have a materially reduced effect on the operational characteristics, i. e., frequency and electrode voltage, of the applicator and there will result a substantial stabilization of the applicator.

This stabilization effect together with compensatory effect of the skirt, ideally expressed as AC1+AC2=O (6) results in still further improved operation of heating applicators; for while the ideal conditions of Equation 6 and where K1 is a constant, may only be approached, the effect of the extent of change in interelectrode capacitance AC1 over and above the extent of compensatory change in skirt capacitance AC2 or AC1+AC2=AC1 (8) is substantially nullified since CMi-l-CM2 AC1' The increased capacitance afforded by the skirt 26, additionally, like the capacitance of the supplemental areas Nth-104 of the aforesaid Warren applications, results in a higher interelectrode voltage.

As stated above, the electrode 25 is supported in spaced relation from all walls of the housing by structure includ ing an inductor 27, 27a one end of which is conductively attached to the electrode 25, intermediate the ends and sides thereof, and the other end of which is conductively attached to the upper horizontal wall 21 of the housing. The inductor 27, 27a provides substantially all the inductance of the resonant applicator. The inductor 27 has blunted corners, a feature claimed in my Patent No. 2,711,468 and is illustrated as being generally similar to the extensible box-fin construction illustrated in my to pending application, Serial No. 263,803, filed December 28, 1951, upon which my said patent issued. Examples of other embodiments of inductances which may be suc cessfully employed are illustrated in my copending application, Serial No. 339,054, filed February 26, 1953, copending application of Foster M. Sweets, Serial No. 339,007, filed February 26, 1953, and in copending application Serial No. 419,070, filed March 26, 1954, which is a continuation-in-part of the aforesaid Warren application Serial No. 13 8,628.

The high-frequency magnetic field circulating about the inductance 27 is created by excitation of the applicator by a coupling loop 28. The loop 28 as illustrated may be in the plate circuit of a high-frequency oscillator tube 30 whose resonant tank circuit is to a large extent comprised of the inductance of fin 27, 27a and a predominant portion of the total capacitance comprised of the heating electrode capacitance and the skirt-sidewall capacitance. The coupling loop which may be formed of copper piping is supported in fixed position within the applicator 10.

Many various types of oscillator circuits may be used for exciting the applicator 1i) herein shown; the particular oscillator circuit 29 illustrated is generically similar to one originally described and claimed in the aforesaid Warren application Serial No. 136,628 In the oscillator system 29, the fixed loop 28 within the metallic housing 20 inductively couples the applicator to the plate or anode circuit of the oscillator tube 30. 111 the particular circuit shown one end of coupling loop 28 is conductively connected to the wall structure 23 of applicator 10 and the other end'of the loop is connected by conductor 31 to the anode of tube 30. The grid of tube 30 is connected to the heating electrode 25 by way of an external variable capacitor 32 and a lead 33' passing through an insulator in the side wall 23 of the housing 20. Alternatively as in the copending Moore application, Serial No. 345,663, filed March 30; 1953, the capacitor 32 may be within the applicator and adjustable concurrently with the movable electrode 25. The cathode of tube 30 so far as the operating frequency of the oscillator is concerned, is grounded through bypass capacitors 34. A direct current source of high voltage B+, B-, is connected between ground and the cathode of tube 30; the positive end of that terminal being grounded as indicated. A direct current path between the grid and cathode of tube 30 is provided by radio frequency choke 35 and grid leak resistor 36. Alternatively the negative terminal of the D. C. source of high voltage may be grounded; in such case there may be employed a wellknown parallel feed arrangement which is not shown but which would include a blocking condenser in series with the coupling loop 28 and a radio frequency choke connected in series between the plate of the tube and the positive terminal of the high voltage supply. Heating for the filaments of the oscillator tube is provided from a source (not shown) by way of terminals H, H.-

In applicators substantially wider than electrode 25, the coupling loop 28 may be fixed in position clear of the path of electrode skirt 26. In narrower applicators, it may be necessary to provide a recess in the electrode skirt for accommodating a portion of the coupling loop.

This construction is illustrated in Figs. 3 and 5 wherein a portion of the electrode skirt 26 has been omitted to provide a recess or gap 26a for accommodating'the leading edge 23a of the coupling loop 28. The electrode skirt 26 is constructed with reenforcing gussets 26b disposed at spaced intervals around its periphery. Care is exercised in positioning the loop with respect to the gap 26:: to provide proper coupling to the applicator and to assure proper voltage clearances.

in the particular applicator illustrated, wherein the dimensions of the electrode are approximately 4 feet x feet, and the electrode spacing is variable over a range of approximately 6 inches to 24 inches an acute angular relationship between the vertical walls 23 and 24 and the lower wall 22 of approximately 7 is employed. This results in the maintenance of substantially constant electrode voltage over a wide variation in electrode spacing or height. it is to be understood, however, that this particular relationship is merely exemplary and that depending upon the characteristics desired in various other types and sizes of applicators, this angular relationship may vary.

To illustrate the improved operational characteristics of applicators embodying the present invention, reference is made to the graph of Fig. 6, wherein the curves A and B respectively represent the variation in capacitance and frequency with electrode height of an applicator embodying the invention. Curves C, D, E and F are representative of characteristics of applicators which do not include the present invention. Curves C and D respectively represent variations in capacitance and frequency of an applicator whose total capacitanceis der-ivedpredominantly from its heating electrode, there being-no supplementary capacitive surfaces provided. CurvesE and F respectively represent variations in capacitance and frequency of an applicator having only supplemental capacity plates mounted on and extending vertically upward from the electrode at right angles thereto, and cooperating with vertical non-inclined housing walls. This latter construction is illustrated in the aforesaid Warren applications Serial Nos. 138,628 and 419,633, Fig. 25 as being the vertical straps Hi4. As is quite apparent, over a preferred operating range in electrode height from 6" to 24", variations in capacitance and frequency are substantially less in the applicator embodying the present invention.

It is to be understood that the operating range set forth may be extended in either direction by changing the physical sizes of the electrodes and housing and that the range set forth is merely exemplary of one commercial embodiment.

While the overall heating effect of dielectric heaters of the type illustrated herein, in which the heating electrodes comprise the capacity of the oscillator tank circuit, is not materially affected by variations in frequency such as normally result from changes in electrode spacing, nevertheless such swings in fundamental frequency may produce harmonics which correspond to one or more natural resonant frequencies of various circuit components such as, for example, the plate loop circuit, the grid circuit or bypass circuits, or elements of such circuits. Resonance in these components may result in power losses or other undesirable operating characteristics. Where the harmonic is the natural resonant frequency of a tunnel mode there may additionally result an uneven heating of the work.

Although with applications embodying the present invention, the capacitance may not be held absolutely constant over the operating range, resulting in some change in frequency and electrode voltage, the slight variation therein has a negligible effect on the operating characteristics of the applicator and thus for all practical purposes the frequency and electrode voltage may be considered as constant.

A preferred embodiment is illustrated in Fig. 5 where except for the gap 260 for accommodating a portion of the coupling loop 23, the inclined electrode skirt 26 defines a continuous surface about the periphery of the electrode 25. The electrode skirt 26 may be comprised of sheet aluminum or other conductive material and is pro vided. with rounded surfaces at all points where the possihility of corona exists. For example, the upper end of the electrode skirt 2% is turned over and all corners are rounded. The electrode skirt 26 is rounded on the gap side and is reenforced by gusset plates 26b spaced about its perimeter. The heating electrode is constructed in like manner of conductive material reenforced by channel members 25a (Figs. 3, 4 and 7). Abrupt changes in the configuration of the electrode such as might exist at the corners thereof are avoided by providing rounded surfaces in prevention of corona.

Variation in electrode height may be effected by any suitable means. The electrode lifting mechanism illus trated is of a type wherein a plurality of lifting rods d9 (Figs. 3, 7) are connected to the upper end of the box-fin element 27 and extend upwardly within the box-fin element 27a to a position above the upper horizontal wall 21 of the housing 29. The lifting rods 49 may embody the well known captive nut principle, or any other construction whereby they will be variable in length to provide for the desired adjustment of the electrode height.

The lifting rods 4h may be adjusted in length by means of a driving means such as an electric motor 41, Fig. 7, which is mechanically coupled to the rods by means of any well known linkage and gearing arrangement. To facilitate movement of the electrode 25 and yet maintain the rods shielded from the magnetic field within the housing 20, the inductor element 27a is formed of accordion pleats which flex with the raising and lowering of the electrode.

In Fig. 7, the applicator 10 is shown to include a conveyor for continuously moving work into the heating housing 20. The conveyor 45 may be of the metallic type serving as a lower heating electrode in place of the lower horizontal wall 22 (Fig. 3). It is well known in the dielectric heating art that the characteristics of the work under treatment vary during the course of travel through the applicator. This variation in work characteristic is reflected in changes in load demand upon the oscillator.

When it is desired to vary the voltage gradient of the electrostatic field along the path of the work the heating electrode 25 may be tilted in the direction of the work to provide for decreasing electrode spacing in the direction of conveyor travel. To that end, the lifting mechanism is constructed so that each pair of lifting rods supporting opposite ends of the electrode 25 may be independently adjusted. One pair of rods is driven by a shaft 42 and the other pair is driven by a shaft 43. The shafts 42 and 43 are connected to their respective rods by gear boxes 42a and 42b and to the motor 41 by couplers 41a and 41b. As each pair of rods is to be adjusted to vary the inclina tion of the electrode 25, its respective shaft is uncoupled from the motor 41 and manually rotated. When the desired angular relationship between the heating electrodes has been established, the shafts 42 and 43 are recoupled to the motor whereupon the electrodes will maintain their angular relationship throughout the range of vertical adjustment.

During the operation of the applicator, vapor released from the drying work may be removed from the confines of the heating chamber, to avoid condensation upon the heating electrode and the treated work, by establishing a current of air entering into the heating chamber by way or" the exit vestibule 13, then passing the work and heating electrode 25 to a port 46 formed in the wall of the housing 29, and finally exiting by way of an exhaust port 50. This stream of air, the path of which is illustrated by arrows, is created by an exhaust fan 51 mounted Within a duct 53 adjacent the outer wall of the applicator 10. The rate of air passing through the heating chamber is regulated by a damper plate 54 mounted within the port 46. Egress of noxious fumes from the heating chamber into the as mosphere outside the applicator is precluded by a second air stream flowing through the duct 53 from the entrance vestibule 32. The rate of air drawn through screened opening 55 of the vestibule is regulated by a second damper plate mounted within the duct 53. The dampers 5dand 56 may be mechanically coupled as by rod 57 in order that the rate of air passing through the housing 26 may be varied Without causing change in the loading on the exhaust fan 51; as one damper moves toward its closed position, the other damper is moved toward its open position.

The applicator is provided with an outer wall 69 which together with the walls of the heating chamber 29 provides an air space M about the heating chamber 2%. This dead air space helps maintain a higher temperature within the worl -hcating chamber or housing and thereby aids in preventing condensation on the electrode and the work.

Access to the heating chamber and the electrode 25 for repair and adjustment purposes is afforded by a hatch or manhole d2 communicating with a passageway 63 through the walls of the applicator.

it shall be understood that the invention is not limited to the embodiments shown and that equivalents thereof are within the scope of the appended claims.

What is claimed is:

l. A dielectric heating applicator comprising an enclosure including conductive horizontal and vertical walls, said vertical walls being inclined and converging in direction away from one of said horizontal walls, a heating electrode disposed within said enclosure substantially parallel to a first of said horizontal walls, fin structure attached at its opposite ends to said heating electrode and to a second of said horizontal walls, said fin structure being extensible to vary the spacing between said heating electrode and said first wall, said enclosure, electrode and fin structure forming a resonator whose resonant wavelength is long compared to the resonator dimensions, and supplemental capacitive elements supported by said heating electrode in parallel relation with said inclined walls and extending therefrom in spaced relation to said fin structure and to all of said walls to minimize change in the total capacitance of said resonator as said fin structure is extended to vary said spacing.

2. A dielectric-heating applicator comprising an electrically conductive housing having a vertical wall, upper and lower heating electrodes, said vertical wall being inclined at an acute angle with said lower heating elec trode and at one end electrically interconnected thereto, means including an inductive element for electrically connecting said upper electrode andan opposite end of said inclined wall, means for effecting vertical movement of said upper electrode to increase or decrease the spacing between it and said lower electrode, and means for minimizing the variation in total capacitance of the applicator as the upper electrode is moved vertically, said means comprising a capacitive surface mounted on said electrode substantially parallel with said inclined vertical wall of said housing.

3. A dielectric-heating applicator as in claim 2 in which said inductive element is an extensible conductive fin and in which said upper electrode is conductively attached to one end of said extensible conductive fin whose opposite end is attached to that part of said housing which is opposite said lower heating electrode, said fin forming the inductance of a resonant tank circuit tuned by said total capacitance.

4. A dielectric-heating applicator as in claim 3 including a coupling loop within said housing, and in which the capacitive surface includes a recess to receive the coupling loop.

5. A dielectric-heating applicator comprising an elec trically conductive housing having a horizontal wall and vertical walls converging in direction away from said horizontal wall, an electrode within said housing having a surface substantially parallel to said horizontal wall and surfaces respectively substantially parallel to said converging walls, an extensible conductive fin having one end conductively attached to said electrode and an opposite end conductively attached to that part of said housing opposite said horizontal wall, said fin forming the inductance of a resonant tank circuit tuned by said electrode, a coupling loop within said housing, one of said electrode surfaces provided with a recess for accommodating said coupling loop, and means for effecting vertical movementof said electrode to increase or decrease the spacing between said horizontal wall and surface to accommodate different loads to be heated and concurrently in opposite sense to vary the spacing between said converging walls and surfaces to minimize the variation in total capacitance of said applicator.

6. In a high-frequency dielectric heating applicator including at least one inclined wall, a pair of spaced heating electrodes having a minimum value of capacitance, conductive structure including said wall electrically interconnecting said electrodes, one of said electrodes being movable, a supplementary capacitive surface mounted on said movable electrode, spaced from and parallel with said inclined wall and having a minimum value of capacitance therewith, and means for raising and lowering said movable electrode vertically to vary the spacing between said electrodes and to vary in opposite sense the spacing between said capacitive surface and inclined wall, the sum total of said minimum capacitances being very much greater than the total change in capacitance between said electrodes to reduce the effect of the change of electrode spacing on applicator operation.

7. In a high-frequency dielectric heating applicator including a conductive housing having at least one inclined wall,a pair of spaced heating electrodes within said housing and having a minimum value of capacitance, means including conductive structure and a portion of the conductive housing for interconnecting said heating electrodes with said inclined wall, one of said electrodes being movable, a supplementary capacitive surface mounted on said movable electrode, spaced from and parallel to said inclined wall and having a minimum value or capacitance, and means for raising and lowering said movable electrode vertically to accommodate work to be heated and for simultaneously varying in opposite sense the spacing between said capacitive surface and said inclined wall, change in the supplementary capacitance being substantially equal and opposite to change in the electrode capacitance providing very small change in applicator capacitance, the sum total of said minimum capacitances being very much greater than the total change in electrode capacitance to thereby nullify the efiect of the small change in applicator capacitance on applicator operation.

8. A high-frequency heating applicator comprising a pair of electrodes supported in spaced relation one from the other, at least one of said electrodes being movable relative to the other, an inductance structure electrically connected at one end to a first one of said electrodes and extending away from said first electrode, a housing enclosing said inductance structure and the space between said electrodes, said housing having conductive walls, structure including said walls electrically interconnecting the other end of said inductance structure with the second of said electrodes, one of said conductive Walls being at least in part inclined at an acute angle relative to the surfaces of said electrodes, means for adjusting said movable electrode to vary the spacing between said electrodes, and a supplemental capacitive element supported by said movable electrode in substantially parallel relation with said inclined part of said one of said conductive walls to minimize change in the total capacitance of said applicator as said movable electrode is adjusted to vary said spacing.

9. A high-frequency heating applicator as in claim 8 in which at least two opposite conductive walls of said housing converge in direction away from said electrodes, and in which supplemental capacitive elements are supported by said movable electrode in substantially parallel relation with said inclined walls to minimize change in the total capacitance of said applicator as the spacing between said electrodes is varied.

10. A high-frequency heating applicator as in claim 8 in which said inductance structure is electrically connected to said movable electrode and is at least in part extensible.

11. A high-frequency heating applicator comprising a pair of electrodes supported in spaced relation one from the other, at least one of said electrodes being movable relative to the other, an inductance structure electrically connected at one end to said movable electrode and extending away therefrom, a housing enclosing said inductance structure and the space between said electrodes, said housing having conductive walls, structure including said walls electrically interconnecting the other end of said inductance structure with the other of said electrodes, at least two opposite conductive walls being at least in part inclined at an acute angle relative to the surfaces of said electrodes and converging in direction away from said electrodes, means for adjusting said movable electrode to vary the spacing between said electrodes, and at least two supplemental capacitive elements supported by said movable electrode in substantially parallel relation wtih said inclined parts of said conductive walls to minimize change in total capacitance of said applicator as said movable electrode is adjusted to vary said spacing.

12. A high-frequency dielectric heating applicator as in claim 8 including a coupling loop within said housing and in which said supplemental capacitive element includes a recess for receiving said coupling loop.

13. A high-frequency heating applicator comprising a pair of electrodes supported in spaced relation one from the other, at least one of said electrodes being movable relative to the other, an inductance structure electrically connected at one end to said movable electrode and extending away therefrom, a housing enclosing said conductive structure and the space between said electrodes, said housing having conductive walls, structure including said walls electrically interconnecting the other end of said inductance structure with the other of said electrodes, said walls being inclined at an acute angle to the surfaces of said electrodes and converging in direction toward the other end of said inductance structure, means for adjusting said movable electrode to vary the spacing between said electrodes, supplemental capacitive elements arranged about the periphery of said movable electrode and each substantially parallel with an opposite housing wall to minimize change in capacitance of said applicator as said movable electrode is adjusted to vary said spacing.

References Cited in the file of this patent UNITED STATES PATENTS 1,580,621 Mahieu Apr. 13, 1926 2,438,476 Dodds Mar. 23, 1948 2,467,782 Schuman Apr. 9, 1949 FOREIGN PATENTS 556,292 Great Britain Sept. 28, 1943 

